Film-screen x-ray imaging devices employing photographic film are widely used for medical imaging. However, the film is often overexposed in some areas and underexposed in other areas due to the limited range of contrast of the film combined with the thickness and composition variations of the tissue across the image. Discrimination of contrast differences of soft tissue in the overexposed and underexposed areas of the film can be difficult. This problem is especially apparent in film-screen mammography.
Attempts have been made at replacing film with electronic image sensors. Potential advantages of electronic image sensors over film include more accurate measurement of x-ray intensity over greater ranges, ability to digitize the image data, ease of archiving and transmitting image data, and improved display capabilities.
However, widespread clinical deployment of digital x-ray radiology has been hampered by the lack of a relatively inexpensive, compact, digital x-ray image sensor of sufficient image size and resolution. Present digital x-ray imaging systems typically use a fluorescing plate that converts each x-ray photon into a large number of visible light photons to produce a visible light image. The visible light image is then imaged onto an optical image sensor such as a CCD. The imaging performance of these techniques is degraded by relatively low x-ray to visible light conversion efficiencies, low collection efficiencies of the light photons, additional quantum noise from the light photons, and loss of resolution due to light spreading in the x-ray to visible light converter.
It is known that selenium is a photoconductive substance, i.e. x-ray photons absorbed in a layer of selenium exposed to an electric field will create a number of electron/hole pairs permitting a current to flow through the otherwise insulating layer. Xerox Corporation developed an x-ray imaging device in which an x-ray induced charge distribution on a selenium-coated aluminum plate is recorded with a paper/toner process. Philips Corporation presently markets a chest x-ray imager in which an x-ray induced charge distribution on a selenium-coated aluminum plate is recorded with scanning electrometers.
Metal oxide semiconductor (MOS) fabrication technology is a well established industry which involves fabrication of integrated circuits on and in the upper surface of a wafer of crystalline silicon. Complimentary metal oxide semiconductor (CMOS) technology combines both n-channel and p-channel transmitters on a single wafer. MOS technology typically utilizes a single crystal silicon substrate as the semiconductor material for transistor fabrication. When we use the term "single crystal silicon substrate" in this application we are referring to a substrate comprised of silicon atoms arranged in a regular crystalline lattice with relatively few defects. The high mobility of charge carriers and well-defined energy bandgap in single crystal silicon results in fast, compact, low-noise circuitry. Other crystalline semiconductor materials are available for use as substrates for metal oxide semiconductor fabrication. These include crystalline silicon on an electrical insulator (such as silicon oxide or sapphire) aluminum gallium, arsenide, and indium gallium arsenide.
Thin film transistor (TFT) technology is an emerging semiconductor fabrication technology in which transistors are fabricated using a thin film of semiconductor material such as amorphous silicon, polycrystalline silicon or amorphous cadmium selenide deposited on an insulating substrate. An advantage of TFT technology is the potential for large area circuits. However, the disordered molecular structure of these thin films leads to low charge mobility and a certain density of localized energy states within the energy bandgap. In comparison with CMOS circuits, TFT circuits are generally slow and noisy with large leakage currents.
Various approaches are presently being proposed and investigated for directly acquiring a digital x-ray image. For example, Zhao and Rowlands (Proc. SPIE 1993; 1896: 114-120) have proposed a readout array fabricated using cadmium selenide TFT technology with an amorphous selenium coating. Tran et. al. disclose, in U.S. Pat. No. 5,235,195, TFT array circuits coated first with a "planarization" layer which in turn is coated with an energy-sensitive layer. A need still exists for improved x-ray imaging devices the present invention provides such a device.